In order to enhance the wear resistance of a tire tread, it is effective to reduce the negative ratio, i.e. the groove area ratio, of a central region of the tread surface (a range, centered on the tire equator, of 50% of the tread surface width, i.e. of the tread contact width given by the linear distance in the tire axial direction between outermost contact edges in the tire axial direction in the tire ground contact area with a flat plate, for a tire mounted on an applicable rim, with specified air pressure and maximum load capability applied, in a vertical position with a camber angle of zero degrees), thereby enhancing the land portion rigidity and suppressing deformation of the land portion. In this case, however, due to a reduction in heat dissipation efficiency of the heated tire tread, heat damage occurs more easily, such as separation of the central region of the tire tread from the peripheral surface of the belt.
Here, an “applicable rim” refers to a rim specified by the standards below in accordance with tire size. “Specified air pressure” refers to air pressure specified by the standards below in accordance with the maximum load capability. The “maximum load capability” refers to the maximum mass that the tire is allowed to bear according to the standards below.
Note that the air referred to here can be substituted with, for example, an inert gas such as nitrogen gas.
The standards are valid industrial standards for the region in which the tire is produced or used, such as “The Tire and Rim Association, Inc. Year Book” in the United States of America, “The European Tyre and Rim Technical Organisation Standards Manual” in Europe, and the “JATMA Year Book” of the Japan Automobile Tire Manufacturers Association in Japan.
Therefore conventionally, the heat dissipation effect of the tread is enhanced by providing circumferential grooves, in the central region of the tread surface, extending continuously in the tread circumferential direction in a straight line, a zigzag pattern, or the like.
Providing circumferential grooves in the central region of the tread surface, however, facilitates width direction deformation when the tire is rotated with load applied thereon, whereby the tread surface deforms inwards in the tread width direction upon leading and returns elastically outwards in the tread width direction upon trailing. In particular, the problem arises that in tires mounted on the front axle of a heavy load vehicle, a large width direction deformation occurs centered on positions that are at ¼ of the tread surface width from the tire equator, causing wear to progress from the ¼ positions.
To address this problem, the circumferential grooves in the central region of the tread surface can be made narrow, as disclosed for example in Patent Literature 1, in order to suppress deformation in the central region and thus improve the wear resistance.